1. Field of the Invention
The present invention relates to an optical disc and a process of making the optical disc.
2. Prior Art Discussion
Optical recording discs of the type accommodating a so called closed space are disclosed in European patent application Nos. 0,109,245, and 0,049,813, and U.K. patent application No. 2,087,129. In the optical recording disc of the above type, a transparent disc substrate is disposed on the outermost part of the recording disc and a spindle insertion hole for receiving a rotation spindle of a disc player is defined in the transparent disc substrate.
German patent application Ser. No. 2,624,375 discloses an improved structure of the optical recording disc in which a central annular member acting as a hub and an outer annular member acting as an information recording substrate are assembled in one body by inserting the central annular member in the central opening of the outer annular member with a flange of the central annular member bonded with the outer annular member. In the arrangement, the spindle hole for receiving the spindle of the disc player is defined in the central portion of the central annular member. However, the German patent application Ser. No. 2,624,375 is directed to a single sided optical recording disc and does not disclose a dual sided optical recording disc. Moreover, there is not disclosed any way of alignment between the centers of the central annular member and the center of the outer annular member.
As the recording substrate it is required to use a material having a good optical property, such as glass without birefringence. In case the spindle insertion hole is opened in the glass recording substrate so that the glass recording substrate can contact a part of hub, there may occur a problem of breakage at the edge portion of the spindle hole.
In the optical recording disc of the type accommodating a closed space comprising two transparent recording substrates which are provided with the spindle insertion holes and separately opposed by an outer spacer and an inner spacer, each disposed at the outer peripheral portion and inner peripheral portion of the recording substrates, when the recording disc is rotated on the disc player by rotation of the spindle of the disc player, there will occur wearing of the peripheral wall of the recording substrate defining the spindle insertion hole and cracking at the bonded portions between the transparent recording substrate and spacer.
In the case of using a hub in the central portion of the transparent disc substrate, it is required that the hub use high abrasion resistance material against the rotational friction between the spindle of the disc player and transparent disc substrate. Moreover, in case a cut portion is formed in the inner peripheral edge of the transparent disc substrate for fitting the flange of the hub so that the flange portion of the hub is bonded to the transparent disc substrate, there may occur cracks in the transparent disc substrate.